The interlayer ordering between ferromagnetic Fe layers in Fe͞V (001) superlattices is switched from initially parallel to antiparallel, as well as antiparallel to parallel, upon introducing hydrogen to the V layers. This process is reversible upon removal of the hydrogen. The results unambiguously prove that the major cause of the interlayer coupling transitions is not the hydrogen-induced changes of the thickness of the V layers, but most likely the distortion of the Fermi surface in the V layers.[S0031-9007(97)03652-1] PACS numbers: 75.70. Cn, 68.55.Ln, 75.50.Bb Since the discovery of the oscillatory magnetic ordering in metallic multilayers [1], substantial work has been devoted to the exploration of the underlying mechanism of the interlayer exchange coupling. For transition metal superlattice systems, current interpretations attribute the basis of the coupling to extremal values of the Fermi wave vector and the discrete lattice spacing of the constituents [2][3][4]. In these models, the coupling between ferromagnetic layers oscillates between parallel and antiparallel alignments as a function of the thickness of an intervening spacer layer. Although a conceptually attractive description of the nature of the exchange coupling is obtained, the complexity of the Fermi surface makes the interpretation far from trivial for the transition metals. The detailed topography of the Fermi surface is expected to play a vital role in the exchange coupling [5]. Experimental exploration of this fundamentally important issue is difficult, since the means to continuously alter the lattice spacing and/or the Fermi energy are limited. One of the first attempts to systematically investigate these effects was on (110) Fe͞Cr 12x V x multilayers [6]. The experimental results were successfully reproduced theoretically, but no experimental data on antiferromagnetic (AFM) coupled Fe͞V (001) multilayers were presented.AFM ordered Fe͞V (001) superlattices have only recently been produced and characterized [7,8]. Samples with three monolayers of Fe were found to be magnetically isotropic in the plane, independent of the V layer thickness [8]. If the electronic states and/or the thickness of the V layers of these samples could be altered continuously, an attractive way of exploring the details of the magnetic interlayer coupling between the ferromagnetic Fe slabs across the nonmagnetic V would be obtained. We will show that this can be done by loading the V layers with H.The hydrogen uptake of Fe͞V superlattices is well known, and the H is found to reside exclusively in the V lattice [9]. The total thickness of the V layers can be changed reversibly by as much as 10% at moderate

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Using polarized neutron reflectometry (PNR) and high angle neutron scattering from Fe/Cr(001) superlattices, we demonstrate how the non-collinear exchange coupling between the Fe layers is caused by a frustration between antiferromagnetic Cr domains. This induces a spiral modulation of the Cr not observed in bulk. PNR and magnetization measurements show that the noncollinear coupling vanishes above the Néel temperature of this commensurate Cr order. The results are consistent with a recent model for non-collinear exchange coupling over antiferromagnetic interlayers.

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To many biophysical characterisation techniques, biological membranes appear as twodimensional structures with details of their third dimension hidden within a 5 nm profile. Probing this structure requires methods able to discriminate multiple layers a few Ångströms thick. Given sufficient resolution, neutron methods can provide the required discrimination between different biochemical components, especially when selective deuteration is employed. We have used stateof-the-art neutron reflection methods, with resolution enhancement via magnetic contrast variation to study an oriented model membrane system. The model is based on the Escherichia coli outer membrane protein OmpF fixed to a gold surface via an engineered cysteine residue. Below the gold is buried a magnetic metal layer which, in a magnetic field, displays different scattering strengths to spin-up and spin-down neutrons. This provides two independent datasets from a single biological sample. Simultaneous fitting of the two datasets significantly refines the resulting model. A β-mercaptoethanol (βME) passivating surface, applied to the gold to prevent protein denaturation, is resolved for the first time as an 8.2 ± 0.6 Å thick layer, demonstrating the improved resolution and confirming that this layer remains after OmpF assembly. The thiolipid monolayer (35.3 ± 0.5 Å), assembled around the OmpF is determined and finally a fluid DMPC layer is added (total lipid thickness 58.7 ± 0.9 Å). The dimensions of trimeric OmpF in isolation (53.6 ± 2.5 Å), after assembly of lipid monolayer (57.5 ± 0.9 Å) and lipid bilayer (58.7 ± 0.9 Å), are precisely determined and show little variation.